A multilayer wired circuit board is formed by laminating a plurality of wired circuit boards, each having an insulating layer and a conductor layer formed in a specific wired circuit pattern on the insulating layer, through an adhesive layer sandwiched therebetween. The conductor layers are electrically connected with each other through conductive passages extending in a thickness direction of the adhesive layers interposed between the conductor layers.
It is typical in the method of producing this multilayer wired circuit board that the wired circuit boards are multilayered in the lamination process shown in FIG. 4, for example. That is to say, a first wired circuit board 3 having a first insulating layer 1 and a first conductor layer 2 formed in a specific wired circuit pattern on the insulating layer is prepared, first, as shown in FIG. 4(a). Then, a thermosetting adhesive layer 5 is laminated on the first conductor layer 2 of the first wired circuit board 3 in its B-stage state, as shown in FIG. 4(b). Then, an opening 4 is formed in the thermosetting adhesive layer 5, as shown in FIG. 4(c). Thereafter, a solder paste 6 is screen-printed on the opening 4, as shown in FIG. 4(d), and then is melted by heating (reflow soldering) to form a solder bump 7a, as shown in FIG. 4(e). Thereafter, a second wired circuit board 10 having a second insulating layer 8 and a second conductor layer 9 formed in a specific wired circuit pattern on the second insulating layer is positioned to be opposed to the solder bump 7a and laminated on the thermosetting adhesive layer 5, as shown in FIG. 4(f). Then, those laminated layers are heated under pressure to cure the thermosetting adhesive layer 5, as shown in FIG. 4(g), and thereby the first wired circuit board 3 and the second wired circuit board 10 are adhesive bonded together.
In this lamination process, when the solder paste 6 is melted by heating, to form the solder bump 7a, the thermosetting adhesive layer 5 in the B-stage state is reactively cured to some extent. As a result of this, when the second wired circuit board 10 is laminated on the thermosetting adhesive layer 5 and then is heated under pressure, fluidity of the thermosetting adhesive layer 5 is reduced, so that a sufficient adhesion strength of the interface between the second wired circuit board 10 including the second conductor layer 9 and the thermosetting adhesive layer 5 cannot be obtained. As a result of this, an interlayer strength between the first wired circuit board 3 including the first conductor layer 2 and the second wired circuit board 10 including the second conductor layer 9 deteriorates, leading to a possible problem of electrical conductive failure between the layers.